The present invention relates to a process for preparing 5,6,7,8-tetrahydro-6-(L-erythro-1',2'-dihydroxypropyl)pterin (hereinafter referred to as "tetrahydrobiopterin"), and more particularly to a process for preparing tetrahydrobiopterin which is rich in the (6R)-form, which comprises catalytically hydrogenating 6-(L-erythro-1',2'-dihydroxypropyl)pterin (hereinafter referred to as "L-erythrobiopterin").
In a living body, it has been well known that (6R)-tetrahydrobiopterin has a very important role as a coenzyme in a catecholamine-serotonin synthesis. Recently, the importance of (6R)-tetrahydrobiopterin has been recognized in the course of the fundamental study thereon. It has been expected that (6R)-tetrahydrobiopterin can be widely used for treatment of patients with Parkinson's disease or depression as well as phenylketonuria which has been conventionally treated with (6R)-tetrahydrobiopterin.
In general, (6R)-tetrahydrobiopterin is synthesized by catalytically hydrogenating L-erythrobiopterin. In accordance with a conventional reaction condition, however, the desired (6R)-form is low in yield and it is difficult to purify, since the (6S)-form which does not exist in nature is by-produced at about 50% of the (6R)-form.
For instance, in Matsuura, "Tanpakushitsu Kakusan Koso" 26, 1394(1981), there is proposed a process in which tetrahydrobiopterin is prepared by catalytically hydrogenating L-erythrobiopterin in 1M hydrochloric acid in the presence of a platinum oxide catalyst. According to the above process, a tetrahydrobiopterin mixture having a (6R)/(6S) ratio (a ratio of the (6R)-form to the (6S)-form of tetrahydrobiopterin, hereinafter the same) of at most 2.23 can only be obtained.
In B. Schircks, J. H. Bieri and M. Viscontini, "Helvetica Chim. Acta" 61, 2731 (1978), there is also proposed a process in which tetrahydrobiopterin is prepared by catalytically hydrogenating L-erythrobiopterin in trifluoroacetic acid in the presence of a platinum oxide catalyst. According to the process, only a tetrahydrobiopterin mixture having a (6R)/(6S) ratio of at most about 2 can be obtained.
As mentioned above, according to the conventional processes the resulting tetrahydrobiopterin mixture has a (6R)/(6S) ratio of at most about 2. Therefore, it (6R)-tetrahydrobiopterin of the natural type is prepared in a high yield on an industrial scale, it is necessary to increase the (6R)/(6S) ratio of a tetrahydrobiopterin mixture. Further, when tetrahydrobiopterin is industrially prepared according to the conventional processes, there is a problem in that an acid stable hydrogenation apparatus is required.
As aforementioned, there have not yet been established any process for industrially preparing (6R)-tetrahydrobiopterin.
An object of the present invention is to provide a process for preparing tetrahydrobiopterin which has an improved (6R)/(6S) ratio, i.e. being rich in (6R)-form of the natural type.